Conventionally, in casting by using a shell mold, there is generally used a shell casting mold, which is formed by using a resin coated sand obtained by kneading a fire-resistant particle, a phenolic novolak resin (binder), and hexamethylenetetramine (hardener), and by hot-forming the resin coated sand into a desired shape.
However, in a casting process by using this kind of casting molds, especially by using a casting mold which has a complex configuration, e.g., a casting mold for casting a molded product such as a cylinder head of an internal combustion system, there is a problem of easily causing a fracture or a crack (hereinafter referred to as “crack” of the casting mold) on the molded product, during the casting process using the casting mold, which leads to an occurrence of the veining on the surface on the casting mold. In addition, there is a recent trend that the configuration of the casting mold is increasingly complex, while a number of vents for gas purging is decreased, so that an occurrence of the gas defect within the casting mold at the time of casting due to the tar originated from a bond has also been a serious problem.
In order to solve the above-described problems, there are proposed methods to use bisphenol A, a residue from a production of bisphenol A, and/or a separated compound or a derivative thereof in the residue, as a raw material of the phenolic novolak resin (see Patent Documents 1 and 2). These methods have sufficiently solved the problem of the veining caused on the surface of the molded product. However, there is newly caused a problem of a gas defect, caused by a large amount of the tar generated at the time of casting.
Meanwhile, automobile makers attempt to take measures, such as providing a casting die with vents, so as to reduce the gas defect. However, if a large amount of the tar is produced, the vents are easily blocked, so that a number of times of maintenances of the vents is increased, which leads to lower the productivity, and aggravate the problem. Therefore, there is highly desired to simultaneously solve these problems.
Patent document 1: JP-A-57-68240
Patent document 2: JP-A-2001-321883